Ships, specially cargo ships, are generally designed considering to include weight of loaded cargos, etc., therefore, a ship in a state of loaded with no or few cargos takes in sea water at a port before departure to keep balance of the ship for the necessity of securing the propeller immersion depth and navigational safety, etc. during no cargo. The water used as ballast is called ships' ballast water. The ships' ballast water is sea water, etc. loaded to a ballast tank at a port when departing the port with no cargo, while the ships' ballast water is discharged when loading cargos at a port.
When filling and discharging ships' ballast water to and from ships moving between loading ports and unloading ports of different environments, there is a concern of adversely affecting ecosystems of coasts by difference of microorganisms included in the ships' ballast water between the loading port and unloading port. Therefore, at the international convention for the management of ships' ballast water, the international treaty for the Control and Management of Ships' Ballast Water and Sediments was adopted in February of 2004 and a treatment of ships' ballast water became obliged.
The standard of ships' ballast water treatment established by the International Maritime Organization (IMO) is that the number of living organisms of 50 μm or greater (mainly zooplankton) included in ships' ballast water to be discharged from ships is less than 10 per 1 m3, the number of living organisms of 10 μm or greater but smaller than 50 μm (mainly phytoplankton) is less than 10 in 1 ml, the number of cholera is less than 1 cfu per 100 ml, the number of Escherichia coli is less than 250 cfu per 100 ml, and the number of Intestinal Enterococci is less than 100 cfu per 100 ml.
To satisfy the treatment standard of ballast water as above, a variety of methods to kill microorganisms in sea water to be filled in a ballast tank have been proposed. For example, the patent article 1 discloses a device for killing microorganisms, etc. by irradiating an ultraviolet ray (UV) after filtering raw water. Also, the patent article 2 discloses a device for killing microorganisms by injecting ozone to ballast water. The patent article 3 discloses a ballast water treatment method of killing microorganisms, etc. by generating electrolytic chlorine by an electrolytic device.
As a method of performing a treatment on microorganisms, etc. in ships' ballast water by using a bactericidal agent, the patent article 4 discloses a ships' ballast water treatment method of killing microorganisms, etc. by adding a chlorine-based bactericidal agent, such as sodium hypochlorite and calcium hypochlorite, to ships' ballast water and securing retention time. Also, the patent articles 5 and 6 disclose treatment devices of ships' ballast water for killing microorganisms, etc. by adding sodium hypochlorite to ships' ballast water. Furthermore, a technique of obtaining a bactericidal agent by generating chlorine dioxide as an oxide from chlorate, hydrogen peroxide and sulfuric acid and a technique of generating peracetic from hydrogen peroxide and acetate and killing with excessive hydrogen peroxide and peracetic have been known.